Table Of ContentIGCSE BIOLOGY SYLLABUS ANSWERS
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APRIL 18, 2016
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IGCSE Biology Syllabus Answers
1‐Characteristics and classification of living organisms
≥ Describe the characteristics of living organisms by defining the terms:
Movement is an action by an organism or part of an organism causing a change of position or place.
Respiration describes the chemical reactions in cells that break down nutrient molecules and release energy for
metabolism.
Sensitivity is the ability to detect or sense stimuli in the internal or external environment and to make appropriate
responses.
Growth is a permanent increase in size and dry mass by an increase in cell number or cell size or both.
Excretion is the removal from organisms of the waste products of metabolism (chemical reactions in cells including
Respiration), toxic materials and substances in excess of requirements.
Nutrition is the taking in of materials for energy, growth and development. Plants require light, carbon dioxide, water
and ions. Animals need organic compounds and ions and usually need water.
Photosynthetic plants are called autotrophs and are usually the first organisms in food chains.
Species is a group of organisms that can reproduce to produce fertile offspring.
Binomial system is an internationally agreed system in which the scientific name of an organism is made up of two parts
showing the genus and the species
≥ State that organisms can be classified into groups by the features that they share
A biologist looks for a natural system of classification using important features which are shared by as large a group as
possible. Birds all have wings, beaks and feathers; there is rarely any doubt about whether a creature is a bird or not. In
other cases it is not so easy. As a result, biologists change their ideas from time to time about how living things should
be grouped. New groupings are suggested and old ones abandoned.
≥ Explain that classification systems aim to reflect evolutionary relationships
By classifying organisms it is also possible to understand evolutionary relationships. Vertebrates all have the presence of
a vertebral column, along with a skull protecting a brain, and a pair of jaws (usually with teeth). By studying the anatomy
of different groups of vertebrates it is possible to gain an insight into their evolution.
≥ Explain that classification is traditionally based on studies of morphology and anatomy
Classification is traditionally based on studies of morphology (the study of the form, or outward appearance, of
organisms) and anatomy (the study of their internal structure, as revealed by dissection).
≥ Define and describe the binomial system
The binomial system of naming species is an internationally agreed system in which the scientific name of an organism is
made up of two parts showing the genus and the species. Binomial means ‘two names’; the first name gives the genus
and the second gives the species. For example, the stoat and weasel are both in the genus Mustela but they are
different species; the stoat is Mustela erminea and the weasel is Mustela nivalis.The name of the genus (the generic
name) is always given a capital letter and the name of the species (the specific name) always starts with a small letter.
Frequently, the specific name is descriptive, for example edulis means ‘edible’, aquatilis means ‘living in water’, bulbosus
means ‘having a bulb’, serratus means ‘having a jagged (serrated) edge’.
≥ Explain that the sequences of bases in DNA and of amino acids in proteins are used as a more accurate
means of classification and explain that organisms which share a more recent ancestor (are more closely
related) have base sequences in DNA that are more similar than those that share only a distant ancestor.
The use of DNA has revolutionized the process of classification. Eukaryotic organisms contain chromosomes made up of
strings of genes. The chemical which forms these genes is called DNA. The DNA is made up of a sequence of bases,
coding for amino acids and, therefore, proteins. Each species has a distinct number of chromosomes and a unique
sequence of bases in its DNA, making it identifiable and distinguishable from other species. This helps particularly when
different species are very similar morphologically (in appearance) and anatomically (in internal structure). The process of
biological classification called cladistics involves organisms being grouped together according to whether or not they
have one or more shared unique characteristics derived from the group’s last common ancestor, which are not present
in more distant ancestors. Organisms which share a more recent ancestor (and are, therefore, more closely related)
have DNA base sequences that are more similar than those that share only a distant ancestor.
≥ List the features in the cells of all living organisms, limited to cytoplasm, cell membrane and DNA as
genetic material
All living organisms have certain features in common, including the presence of cytoplasm and cell membranes, and DNA
as genetic material.
≥ List the features in the cells of all living organisms, limited to ribosomes for protein synthesis and enzymes
involved in respiration
All living organisms also contain ribosomes in the cytoplasm, floating freely or attached to membranes called rough
endoplasmic reticulum (ER). Ribosomes are responsible for protein synthesis.
≥ List the main features used to place all organisms into one of the five kingdoms: Animal, Plant, Fungus,
Prokaryote, and Protoctist.
The Animal kingdom
Animals are multicellular organisms whose cells have no cell walls or chloroplasts. Most animals ingest solid food and
digest it internally.
Arthropods
The name arthropod means ‘jointed limbs’, and this is a feature common to them all. They also have a hard, firm
external skeleton, called a cuticle, which encloses their bodies. Their bodies are segmented and, between the segments,
there are flexible joints which permit movement. In most arthropods, the segments are grouped together to form
distinct regions, the head, thorax and abdomen.
1. Crustacea
Like all arthropods, crustacea have an exoskeleton and jointed legs. They also have two pairs of antennae which are
sensitive to touch and to chemicals, and they have compound eyes. Compound eyes are made up of tens or hundreds of
separate lenses with light‐sensitive cells beneath. They are able to form a crude image and are very sensitive to
movement. Typically, crustacea have a pair of jointed limbs on each segment of the body, but those on the head
segments are modified to form antennae or specialised mouth parts for feeding.
2. Insects
Insects have segmented bodies with a firm exoskeleton, three pairs of jointed legs, compound eyes and, typically, two
pairs of wings. The segments are grouped into distinct head, thorax and abdomen regions. Insects differ from crustacea
in having wings, only one pair of antennae and only three pairs of legs. There are no limbs on the abdominal segments.
The insects have very successfully colonised the land. One reason for their success is the relative impermeability of their
cuticles, which prevents desiccation even in very hot, dry climates.
3. Arachnids
Their bodies are divided into two regions, the cephalothorax and the abdomen. They have four pairs of limbs on the
cephalothorax, two pedipalps and two chelicerae. The pedipalps are used in reproduction; the chelicerae are used to
pierce their prey and paralyse it with a poison secreted by a gland at the base. There are usually several pairs of simple
eyes.
4. Myriapods
They have a head and a segmented body which is not obviously divided into thorax and abdomen. There is a pair of legs
on each body segment but in the millipede the abdominal segments are fused in pairs and it looks as if it has two pairs of
legs per segment. As the myriapod grows, additional segments are formed. The myriapods have one pair of antennae
and simple eyes.
Vertebrates
Vertebrates are animals which have a vertebral column. The vertebral column is sometimes called the spinal column or
just the spine and consists of a chain of cylindrical bones (vertebrae) joined end to end. So‐called ‘warm‐blooded’
animals, for the most part, have a body temperature higher than that of their surroundings. The main difference,
however, is that these temperatures are kept more or less constant despite any variation in external temperature. There
are internal regulatory mechanisms (see Chapter 14) which keep the body temperature within narrow limits. It is better
to use the terms poikilothermic (variable temperature) and homoiothermic (constant temperature). However, to
simplify the terms, ‘cold blooded’ and ‘warm blooded’ will be referred to in this section.
1. Fish
Fish are poikilothermic (cold blooded) vertebrates. Many of them have a smooth, streamlined shape which offers
minimal resistance to the water through which they move (see Figure 1.16). Their bodies are covered with overlapping
scales and they have fins which play a part in movement. Fish breathe by means of filamentous gills which are protected
by a bony plate, the operculum. Fish reproduce sexually but fertilisation usually takes place externally; the female lays
eggs and the male sheds sperms on them after they have been laid.
2. Amphibia
Amphibia are poikilothermic (cold blooded) vertebrates with four limbs and no scales. The class includes frogs, toads
and newts. The name, amphibian, means ‘double life’ and refers to the fact that the organism spends part of its life in
water and part on the land. In fact, most frogs, toads and newts spend much of their time on the land, in moist
situations, and return to ponds or other water only to lay eggs.
Extensions (Difference between newts, frogs and toads)
Amphibia have four limbs. In frogs and toads, the hind feet have a web of skin between the toes. This offers a large
surface area to thrust against the water when the animal is swimming. Newts swim by a wriggling, fish‐like movement of
their bodies and make less use of their limbs for swimming. Amphibia have moist skins with a good supply of capillaries
which can exchange oxygen and carbon dioxide with the air or water. They also have lungs which can be inflated by a
kind of swallowing action. They do not have a diaphragm or ribs. Frogs and toads migrate to ponds where the males and
females pair up. The male climbs on the female’s back and grips firmly with his front legs. When the female lays eggs,
the male simultaneously releases sperms over them. Fertilisation, therefore, is external even though the frogs are in
close contact for the event.
3. Reptiles
Reptiles are land‐living vertebrates. Their skins are dry and the outer layer of epidermis forms a pattern of scales. This
dry, scaly skin resists water loss. Also the eggs of most species have a tough, parchment like shell. Reptiles, therefore,
are not restricted to damp habitats, nor do they need water in which to breed. Reptiles are poikilothermic (cold
blooded) but they can regulate their temperature to some extent. They do this by basking in the sun until their bodies
warm up. When reptiles warm up, they can move about rapidly in pursuit of insects and other prey.
4. Birds
Birds are homoiothermic (warm blooded) vertebrates. The vertebral column in the neck is flexible but the rest of the
vertebrae are fused to form a rigid structure. This is probably an adaptation to flight, as the powerful wing muscles need
a rigid frame to work against. The epidermis over most of the body produces a covering of feathers but, on the legs and
toes, the epidermis forms scales. The feathers are of several kinds. The fluffy down feathers form an insulating layer
close to the skin; the contour feathers cover the body and give the bird its shape and colouration; the large quill feathers
on the wing are essential for flight. Birds have four limbs, but the forelimbs are modified to form wings. The feet have
four toes with claws which help the bird to perch, scratch for seeds or capture prey, according to the species. The upper
and lower jaws are extended to form a beak which is used for feeding in various ways. In birds, fertilisation is internal
and the female lays hard‐shelled eggs in a nest where she incubates them.
5. Mammals
Mammals are homoiothermic (warm blooded) vertebrates with four limbs. They differ from birds in having hair rather
than feathers. Unlike the other vertebrates they have a diaphragm which plays a part in breathing. They also have
mammary glands and suckle their young on milk. Humans are mammals. All mammals give birth to fully formed young
instead of laying eggs. The eggs are fertilized internally and undergo a period of development in the uterus. In either
case, the youngster’s first food is the milk which it sucks from the mother’s teats. The milk is made in the mammary
glands and contains all the nutrients that the offspring need for the first few weeks or months, depending on the
species. As the youngsters get older, they start to feed on the same food as the parents. In the case of carnivores, the
parents bring the food to the young until they are able to fend for themselve.
The plant kingdom
It is useful to have an overview of the classification of the plant kingdom, although only two groups (ferns and flowering
plants) will be tested in the examination.
1. Ferns
Ferns are land plants with quite highly developed structures. Their stems, leaves and roots are very similar to those of
the flowering plants. The stem is usually entirely below ground and takes the form of a structure called a rhizome. In
bracken, the rhizome grows horizontally below ground, sending up leaves at intervals. The roots which grow from the
rhizome are called adventitious roots. This is the name given to any roots which grow directly from the stem rather than
from other roots. The stem and leaves have sieve tubes and water conducting cells similar to those in the xylem and
phloem of a flowering plant (see Chapter 8). For this reason, the ferns and seed‐bearing plants are sometimes referred
to as vascular plants, because they all have vascular bundles or vascular tissue. Ferns also have multicellular roots with
vascular tissue. The leaves of ferns vary from one species to another, but they are all several cells thick. Most of them
have an upper and lower epidermis, a layer of palisade cells and a spongy mesophyll similar to the leaves of a flowering
plant. Ferns produce gametes but no seeds. The zygote gives rise to the fern plant, which then produces single‐celled
spores from numerous sporangia (spore capsules) on its leaves. The sporangia are formed on the lower side of the leaf
but their position depends on the species of fern. The sporangia are usually arranged in compact groups.
2. Flowering plants
Flowering plants reproduce by seeds which are formed in flowers. The seeds are enclosed in an ovary. Flowering plants
are divided into two subclasses: monocotyledons and dicotyledons. Monocotyledons (monocots for short), are flowering
plants which have only one cotyledon in their seeds. Most, but not all, monocots also have long, narrow leaves (e.g.
grasses, daffodils, and bluebells) with parallel leaf veins.
The dicotyledons (dicots for short), have two cotyledons in their seeds. Their leaves are usually broad and the leaf veins
form a branching network.
The fungi kingdom
Most fungi are made up of thread‐like hyphae, rather than cells, and there are many nuclei distributed throughout the
cytoplasm in their hyphae. There are also the less obvious, but very important, mould fungi which grow on stale bread,
cheese, fruit or other food. Many of the mould fungi live in the soil or in dead wood. The yeasts are single‐celled fungi
similar to the moulds in some respects.
The Prokaryote kingdom
These are the bacteria and the blue‐green algae. They consist of single cells but differ from other single‐celled organisms
because their chromosomes are not organized into a nucleus.
Bacterial structure
Bacteria (singular: bacterium) are very small organisms consisting of single cells rarely more than 0.01 mm in length.
They can be seen only with the higher powers of the microscope. Their cell walls are made, not of cellulose, but of a
complex mixture of proteins, sugars and lipids. Some bacteria have a slime capsule outside their cell wall. Inside the cell
wall is the cytoplasm, which may contain granules of glycogen, lipid and other food reserves.
Each bacterial cell contains a single chromosome, consisting of a circular strand of DNA (see Chapter 4 and
‘Chromosomes, genes and proteins. The chromosome is not enclosed in a nuclear membrane but is coiled up to occupy
part of the cell.
Individual bacteria may be spherical, rod‐shaped or spiral and some have fi laments, called flagella, projecting from
them. The flagella can flick and so move the bacterial cell about.
The Protoctist kingdom
These are single‐celled (unicellular) organisms which have their chromosomes enclosed in a nuclear membrane to form
a nucleus. Some of the protoctista, e.g. Euglena, possess chloroplasts and make their food by photosynthesis. These
protoctista are often referred to as unicellular ‘plants’ or protophyta. Organisms such as Amoeba and Paramecium take
in and digest solid food and thus resemble animals in their feeding. They may be called unicellular ‘animals’ or protozoa.
Amoeba is a protozoan which moves by a fl owing movement of its cytoplasm. It feeds by picking up bacteria and other
microscopic organisms as it goes. Vorticella has a contractile stalk and feeds by creating a current of water with its cilia.
The current brings particles of food to the cell. Euglena and Chlamydomonas have chloroplasts in their cells and feed,
like plants, by photosynthesis.
Viruses
There are many different types of virus and they vary in their shape and structure. All viruses, however, have a central
core of RNA or DNA (see Chapter 4) surrounded by a protein coat. Viruses have no nucleus, cytoplasm, cell organelles or
cell membrane, though some forms have a membrane outside their protein coats. Virus particles, therefore, are not
cells. They do not feed, respire, excrete or grow and it is debatable whether they can be classed as living organisms.
Viruses do reproduce, but only inside the cells of living organisms, using materials provided by the host cell. The nucleic
acid core is a coiled single strand of RNA. The coat is made up of regularly packed protein units called capsomeres each
containing many protein molecules. The protein coat is called a capsid.
2‐Organisation of the organism
≥ Describe and compare the structure of a plant cell with an animal cell, as seen under a light microscope,
limited to cell wall, nucleus, cytoplasm, chloroplasts, vacuoles and location of the cell membrane
≥ State the functions of the structures seen under the light microscope in the plant cell and in the animal
cell
≥ State that the cytoplasm of all cells contains structures, limited to ribosomes on rough endoplasmic
reticulum and vesicles
Organelles present include the rough endoplasmic reticulum, a network of flattened cavities surrounded by a
membrane, which links with the nuclear membrane. The membrane holds ribosomes, giving its surface a rough
appearance. Rough endoplasmic reticulum has the function of producing, transporting and storing proteins. Ribosomes
can also be found free in the cytoplasm. They build up the cell’s proteins.
All living organisms also contain ribosomes in the cytoplasm, floating freely or attached to membranes called rough
endoplasmic reticulum (ER). Ribosomes are responsible for protein synthesis
≥ State that almost all cells, except prokaryotes, have mitochondria and rough endoplasmic reticulum, state
that aerobic respiration occurs in mitochondria and also state that cells with high rates of metabolism
require large numbers of mitochondria to provide sufficient energy.
Mitochondria are tiny organelles, which may appear slipper‐shaped, circular or oval when viewed in section. In three
dimensions, they may be spherical, rod‐like or elongated. They have an outer membrane and an inner membrane with
many inward‐pointing folds. Mitochondria are most numerous in regions of rapid chemical activity and are responsible
for producing energy from food substances through the process of aerobic respiration. Note that prokaryotes do not
possess mitochondria or rough endoplasmic reticulum in their cytoplasm.
≥ Specialised cells
Most cells, when they have finished dividing and growing, become specialised. When cells are specialised: l they do one
particular job l they develop a distinct shape l special kinds of chemical change take place in their cytoplasm. The
changes in shape and the chemical reactions enable the cell to carry out its special function. Red blood cells and root
hair cells are just two examples of specialised cells.
The specialisation of cells to carry out particular functions in an organism is sometimes referred to as ‘division of labour’
within the organism.
1. Ciliated cells
These cells form the lining of the nose and windpipe,
and the tiny cytoplasmic ‘hairs’, called cilia, are in a
continual flicking movement which creates a stream of
fluid (mucus) that carries dust and bacteria through
the bronchi and trachea, away from the lungs.
2. Root hair cells
These cells absorb water and mineral salts from the soil. The hair‐
like projection on each cell penetrates between the soil particles
and offers a large absorbing surface. The cell membrane is able to
control which dissolved substances enter the cell
3. Xylem vessels
These cells transport mineral ions from the roots to the leaves. A substance called
lignin impregnates and thickens the cell walls making the cells very strong and
impermeable. This gives the stem strength. The lignin forms distinctive patterns in
the vessels – spirals, ladder shapes, reticulate (net‐like) and pitted. Xylem vessels are
made up of a series of long xylem cells joined end‐to‐end. Once a region of the plant
has stopped growing, the end walls of the cells are digested away to form a
continuous, fine tube. The lignin thickening prevents the free passage of water and
nutrients, so the cytoplasm in the cells dies. Effectively, the cells form long, thin,
strong straws.
4. Palisade mesophyll cells
These are found underneath the upper epidermis of plant
leaves. They are columnar (quite long) and packed with
chloroplasts to trap light energy. Their function is to make
food for the plant by photosynthesis using carbon dioxide,
water and light energy.
5. Nerve cells
These cells are specialised for conducting electrical impulses
along the fiber, to and from the brain and spinal cord. The fibers
are often very long and connect distant parts of the body to the
CNS, e.g. the foot and the spinal column. Chemical reactions
cause the impulses to travel along the fiber.
6. Red blood cells
These cells are distinctive because they have no nucleus when mature. They
are tiny disc‐like cells which contain a red pigment called haemoglobin. This
readily combines with oxygen and their function is the transport of oxygen
around the body.
7. Sperm cell
Sperm cells are male sex cells. The front of the cell is oval shaped and
contains a nucleus which carries genetic information. There is a tip, called an
acrosome, which secretes enzymes to digest the cells around an egg and the
egg membrane. Behind this is a mid‐piece which is packed with
mitochondria to provide energy for movement. The tail moves with a whip‐
like action enabling the sperm to swim. Their function is reproduction,
achieved by fertilising an egg cell.
8. Egg cell
Egg cells (ova, singular: ovum) are larger than sperm cells
and are spherical. They have a large amount of cytoplasm,
containing yolk droplets made up of protein and fat. The
nucleus carries genetic information. The function of the egg
cell is reproduction.
≥ Define tissue
A group of cells with similar structures, working together to perform a shared function.
≥ Define organ
A structure made up of a group of tissues, working together to perform specific functions.
≥ Define organ system
A group of organs with related functions, working together to perform body functions.
Description:List the features in the cells of all living organisms, limited to cytoplasm, cell membrane and DNA as genetic material. All living organisms have certain
